Currently I have the option to purchase the above CPU for $100.. + $100 for a Mobo. This would be a replacement for a Phenom 2 965 3.4ghz OC'd up to 3.9ghz.

Would there be any benefit if I overclocked the AMD FX 8120 up to 4+ ghz?

That depends on what you do, motherboard you are buying etc. If you do 3d rendering it would be a major upgrade, for gaming -> meh, not much there to gain since most games use less then 4 cores or half of what the 8120 has. If you do virtualization alot or multitask, hell yeah, great improvement. If for busines type uses, internet etc. not much there. If you do video work or other type of multimedia stuff yes.

In other words not enough info in your usage to determine a good answer for you. To OC well you will need a good HSF combo or water cooler. My cpu overheated with stock 8120 cooler at 4ghz in an A/C cooled case . My 8120 has gone up to 5.3ghz, 5ghz stable (couldn't test higher due to heat issues, my Antec 620 water cooler is not enough to keep it cool past 5ghz and is not really usable at 5ghz either with my A/C rig setup and Antec 620 water cooler. I keep mine around 4.7ghz, solid as a rock, max usable for my setup is around 4.8ghz. If I go with a better water cooler, like the 960 Antec I should be able to go 5ghz+ but I don't see that much gain if did.

I was going to get one but the power usage it has compared to other products has put me off to it. I was going to wait for the Piledriver version of the FX but decided to not wait it out since there is no sure fire way to know if it will be good enough.

If you don't mind the power usage when overclocked and/or will use the 8 cores then it's a great deal.